Spiders produce a number of silks for different functions and are therefore useful organisms to produce a variety of structural proteins. The structural fibers of the golden orb-weaver spider (Nephila clavipes), are extremely strong and flexible, and are able to absorb impact energy from flying insects without breaking. Dragline silk fibers dissipate energy over a broad area and balance stiffness, strength and extensibility. In addition, silk proteins have very low antigenicity. Therefore, silk fibers are well suited for light weight, high performance fiber, composite and medical applications. The composition of these proteins is mainly glycine, alanine, and other short side chain amino acids, which form anti-parallel beta-pleated sheets by hydrogen bonding and hydrophobic interactions; Lucas et al., Discovery 25:19 1964. Many spider silks are resistant to digestion by proteolytic enzymes; Tillinghast and Kavanaugh, Journal of Zoology 202:212 1977, and insoluble in dilute acids and bases; Mello et al., American Chemical Society Symposium Series 544, Silk Polymers: Materials Science and Biotechnology pp 67-79, 1995. Spiders are not capable of producing sufficient quantities of proteins to enable a practical use of their potential. To solve this problem, recombinant spider silks have been expressed in E. coli; Arcidiacono et al., Applied Microbiology and Biotechnology 49:31 1998; Fahnestock and Irwin, Applied Microbiology and Biotechnology 47:23, 1997; Fahnestock and Irwin, Applied Microbiology and Biotechnology 47:33 1997; Lewis et al., Protein Expression and Purification 7:400, 1996; Prince et al., Biochemistry 34:10879 1995. However, the purification and preparation of a protein for fiber spinning has been particularly difficult due to the solubility characteristics and unique properties of spider silk and other structural proteins.
Native Nephila clavipes spider dragline fiber has been partially solubilized in hexafluoroisopropanol (HFIP) and dried to a film. A 2.5% (w/w) solution of the film in HFIP was used for spinning; Jelinski et al., Macromolecules 31:6733 1998. The spinning was conducted with a syringe pump at 6 uL/s by forcing the HFIP solution through the spinneret into a coagulation bath.
Affinity chromatography has been used for purification by binding to an engineered tag in the recombinant protein while washing away bacterial proteins; Arcidiacono et al., Applied Microbiology and Biotechnology 49:31 1998; Fahnestock and Irwin, Applied Microbiology and Biotechnology 47:23 1997; Lewis et al., Protein Expression and Purification 7:400 1996; Prince et al., Biochemistry 34:10879 1995. One commonly used tag is a hexa-histidine tag, that binds with high affinity to a nickel affinity resin. After washing away the bacterial proteins, the tagged recombinant protein can be eluted from the resin. There are several disadvantages to this method: 1) large volumes of denaturing buffers are required, involving multiple steps and time; 2) not all target protein is recovered; 3) other bacterial proteins remain, often requiring additional purification (i.e., high-performance liquid chromatography (HPLC)); 4) the method is not easily scaled-up; 5) and the presence of an affinity tag on the recombinant protein may increase its antigenicity and interfere with the necessary molecular alignment required for high strength fibers. Accordingly, there is a continuing need to develop new methods for the purification of structural proteins, spinning of silk fibers lacking the engineered tag and enabling the assembly of macromolecular structures without potential interferences.